Abstract: A cylinder lock mechanism of conventional type, comprising a fixed outer cylinder and an inner rotatable plug, to be mounted on a door frame, is protected by a hollow cylindrical steel casing which is fitted onto the outer cylinder of said lock mechanism. The cylindrical casing is partially closed in correspondence of the end of the cylinder lock mechanism presenting the keyhole for the insertion of the key, while it is open at the other end, which is the end directed towards the inside of the door. The outer cylinder of the cylinder lock mechanism and the protective casing are secured to the door frame independently from each other, and the outer cylinder can, if desired, rotate within the cavity of the protective casing. An additional protective disc provided with a slot for the insertion of the key into the keyhole is inserted between the front end of the cylinder lock mechanism and the internal side of the partially closed end of the casing.